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A Bash comment is a lexical construct that instructs the shell interpreter to ignore specific text during the parsing and execution phases. A comment is initiated by an unquoted hash character (#) appearing at the beginning of a word, and it terminates unconditionally at the next newline character (\n). Backslashes inside a comment do not escape the newline; they are treated as literal characters. When the Bash parser encounters a # at the start of a line or preceded by unquoted whitespace or metacharacters, it treats that character and all subsequent characters on the same physical line as a single token to be discarded before command execution.

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Syntax and Scope

Comments can be placed at the beginning of a line or inline following a command. 

# This is a full-line comment. The parser ignores this entire line.
command_name --flag  # This is an inline comment. The parser ignores text after the hash.

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Lexical Exceptions

The # symbol loses its comment-initiating property and is treated as a literal character if it is enclosed in single quotes ('), double quotes ("), escaped with a backslash (\), or if it appears anywhere other than the beginning of a word. 
echo "The # symbol here is parsed as part of the string literal."
echo 'Another literal # character.'
echo file#name.txt  # The first # is literal because it is not at the beginning of a word.
echo \# # The first # is escaped and literal. The second # starts the comment.

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Multi-line Comments

Bash does not possess a native block-comment operator (such as /* ... */ found in C-family languages). To span comments across multiple lines, the standard approach is to prefix every line with the # character. 

# Line one of the comment block

# Line two of the comment block

# Line three of the comment block
To bypass prefixing every line, developers often simulate block comments by combining the null utility (:), which does nothing and returns a 0 exit status, with a Here Document (<<). 
: <<'EOF'
The shell parses this block as a string literal passed to the null command.
Because the null command ignores its standard input, this text is effectively
treated as a multi-line comment. Quoting the 'EOF' delimiter prevents parameter 
expansion within the block.
EOF

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The Shebang (#!) Directive

If the # character is immediately followed by an exclamation mark (!) at byte zero (the absolute beginning) of a script file, it forms a magic number (0x23 0x21) known as a shebang. While syntactically resembling a comment to the Bash interpreter, the shebang is a directive read by the operating system’s program loader to determine which interpreter to use for executing the file. 
#!/usr/bin/env bash

# The line above is a shebang directive, not a standard comment.
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